Anion exchange membranes and polymers for use in same

ABSTRACT

Embodiments of the invention relate generally to anion exchange membranes and, more particularly, to anion exchange membranes comprising a styrene block copolymer and methods for their manufacture. In one embodiment, the invention provides a polymer according to formula IV, wherein x and y are mol %, QA is or each of R 1  and R 2  is, independently, a linear alkyl chain or a cyclic alkyl chain, and Z is selected from a group consisting of: a linear alkyl chain, a cyclic alkyl chain, and an alkylene ether chain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of co-pending U.S. ProvisionalPatent Application No. 62/027,497, filed 22 Jul. 2014, which is herebyincorporated herein as though fully set forth.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant number0747667 awarded by the National Science Foundation. The government hascertain rights in the invention.

BACKGROUND

Alkaline exchange membranes or anion exchange membranes (AEMs) allow forthe transportation of anions (e.g., OH⁻, Cl⁻, Br⁻) from the cathode tothe anode in an electrochemical reaction. AEMs are a critical componentof AEM fuel cells, where hydrogen and oxygen are used to generateelectricity, with water as a byproduct. AEMs are also used in waterelectrolysis, where water is split into hydrogen and oxygen usingelectricity. In both AEM fuel cells and water electrolysis, hydroxideions (OH⁻) are transported through the AEM, along with water molecules.AEMs may also be used, for example, in batteries, sensors, and asactuators.

Known AEMs are generally unsuitable for use in AEM fuel cells or waterelectrolysis. Many commercially-available AEMs are based on polystyrene,which is generally considered a poor choice for AEM fuel cells or waterelectrolysis. Other AEM materials contain an arylene ether linkage (—O—)in the mid-chain and a benzyltrimethyl ammonium group in the side-chain.This combination, however, has been found to be chemically unstable andto degrade easily under highly alkaline conditions.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, the invention provides a polymer according to formulaIX

wherein x and y are mol % and n is 1-10.

In another embodiment, the invention provides a polymer according toformula XII

wherein x and y are mol % and n is 1-10.

In still another embodiment, the invention provides a polymer accordingto formula IV

wherein x and y are mol % and n is 1-10, QA is

each of R₁ and R₂ is, independently, a linear alkyl chain or a cyclicalkyl chain, and Z is selected from a group consisting of: a linearalkyl chain, a cyclic alkyl chain, and an alkylene ether chain.

In yet another embodiment, the invention provides an anion exchangemembrane comprising at least one polymer selected from a groupconsisting of:

a polymer of formula IV

wherein x and y are mol % and n is 1-10, QA is

each of R₁ and R₂ is, independently, a linear alkyl chain or a cyclicalkyl chain, and Z is selected from a group consisting of: a linearalkyl chain, a cyclic alkyl chain, and an alkylene ether chain;

a polymer of compound IX

wherein x and y are mol % and n is 1-10; and

a polymer of compound XII

wherein x and y are mol % and n is 1-10.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various embodiments of the invention, in which:

FIG. 1 shows chemical reactions included in an illustrative method ofpreparing a polymer according to an embodiment of the invention;

FIG. 2 shows examples of various amine groups that may be incorporatedinto polymers according to various embodiments of the invention;

FIG. 3 shows chemical reactions included in a method of preparing apolymer according to another embodiment of the invention; and

FIG. 4 shows chemical reactions included in a method of preparing apolymer according to still another embodiment of the invention.

It is noted that the drawings of the invention are not to scale. Thedrawings are intended to depict only typical aspects of the invention,and therefore should not be considered as limiting the scope of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention include a new class of quaternized ammoniumhydroxide-containing polymers prepared from a styrene-butadiene blockcopolymer (SEBS). This new class of polymers may be used in alkalineexchange membranes (AEMs), lack an arylene ether linkage in the polymermain-chain, and can prepared with any of a number of quaternizedammonium groups in the polymer side-chains.

FIG. 1 shows chemical reactions involved in a method of forming aquaternized ammonium hydroxide-containing polymer from an SEBS. An SEBS,compound I, is employed where x and y are mol % of each repeating unitand 2x+y=100. For example, in some embodiments of the invention, x is 15and y is 70. Other values are possible, of course, as will be recognizedby one skilled in the art. An iridium-catalyzed borylation is thenperformed using bis(pinacolato)diboron (B₂Pin₂) to introduce a boronicester group into the aromatic rings of the SEBS, yielding compound II.

A palladium-catalyzed Suzuki coupling reaction is then carried out withan aryl bromide-containing amine to yield compound III. Various aminegroups may be substituted for the boronic ester group in compound II,depending on the aryl bromide-containing amine employed. For example,the R group of the aryl bromide-containing amine may be of formula V orformula VI below, wherein each of R₁ and R₂ is, independently, a linearalkyl chain or a cyclic alkyl chain, and Z is selected from a groupconsisting of: a linear alkyl (e.g., —(CH₂)_(n)—) chain, a cyclic alkylchain, and an alkylene ether chain (e.g., —(CH₂CH₂O)_(n)—CH₂CH₂—).

The resulting polymer is then cast into a film, followed by methylationof the amine groups in the polymer and an ion exchange reaction to formcompound IV, a quaternary ammonium-containing styrene block copolymer(SEBS-QA) according to one embodiment of the invention. Applicants havefound the SEBS-QAs of the invention to be chemically stable and suitablefor use in AEMs, even in highly alkaline environments.

Any of a number of quaternary ammonium groups may be incorporated intothe SEBS-QAs of the invention, some of which may be sterically hinderedand chemically stable. FIG. 2 shows five illustrative R groups and theresulting quaternary ammonium groups. Illustrative quaternary ammoniumgroups include benzyltrimethylammonium (TMA), dimethylpiperazinium(DMP), benzyldicyclohexylmethylammonium (MCH),benzyldiisopropylmethylammonium (MiPr), trimethylhexylammonium (TMHA),and benzyldimethylhexylammonium (DMHA). Other quaternary ammonium groupsmay similarly be employed, as will be recognized by one skilled in theart, and are within the scope of the invention. Other suitablequaternary ammonium groups include, for example, alkyl-substitutedimidazoliums and alkyl-substituted guadiniums.

Table 1 below shows comparative properties of SEBS-QAs according toembodiments of the invention.

TABLE 1 Representative data of SEBS-QA. Theor. Cl⁻ σ at HCO₃ ⁻ σ Mol %IEC Expt. IEC WU 30° C. at 30° C. OH− σ (mS/cm) Sample of amine (meq/g)(meq/g) (%) (mS/cm) (mS/cm) 30° C. 60° C. SEBS-TMA 13 1.52 1.49 211 1323 45 89 SEBS-DMP 13 1.39 1.20 249 10 13 19 33 SEBS-MCH 13 1.25 0.60 522 1 3 8 SEBS-MiPr 12.1 1.33 1.38 49 2 2 4 10 SEBS-TMHA 13 1.36 1.16 2369 22 39 59 SEBS-DMHA 13 1.37 0.99 91 3 4 10 22

The results in Table 1 show that these SEBS-QAs have high anionconductivity (Cl⁻, HCO₃ ⁻, OH⁻), which allows them to be used in a solidelectrolyte membrane (in this case AEM) and as an ionomer in theelectrodes in electrochemical devices.

Because SEBS exhibits a nano-scale phase-separated morphology, theSEBS-QAs of the invention will similarly exhibit nano-scaleion-transporting channels, allowing for the highly effective conductionof ions.

According to other embodiments of the invention, SEBS-based anionexchange membranes may be prepared without the use of an expensivetransition metal catalyst such as iridium or palladium. FIGS. 3 and 4show examples of such methods.

The inset within FIG. 3 shows the reactions involved in theesterification of 6-bromohexanoic acid and its subsequent methylation toa tertiary alcohol, which may be used in preparation of a polymeraccording to the invention. At each stage within the inset, n may be anyinteger value, although n is typically 1-10 (e.g., 5).

Referring now to the inset of FIG. 3, 6-bromohexanoic acid (compound 1)(0.50 g, 2.56 mmol), methanol (3.8 mL), and concentrated sulfuric acid(0.04 mL) are added to a 25 mL round bottom flask and the mixturestirred at 55° C. for 14 hours. Methanol is then evaporated, e.g., usinga rotary evaporator, and the remaining product is diluted with ethylacetate (15 mL), washed with NaHCO₃ (3×10 mL), and dried over Na₂SO₄.Ethyl acetate is then removed, e.g., with rotary evaporation, and theproduct is vacuum dried, yielding the methyl 6-bromohexanoate ester(compound 2). It should be noted that alcohols other than methanol(e.g., ethanol, propanol) can be used for preparation of similar estercompounds.

The methyl 6-bromohexanoate ester (4.8 g, 23.0 mmol) and anhydrous THF(20 mL) are added to a 100 mL round bottom flask under nitrogen andcooled in an ice bath. A mixture of methyl magnesium bromide [(CH₃MgBr,3 M in ether) 23 mL, 69 mmol] in anhydrous THF (10 mL) is then added tothe flask, e.g., by syringe. The ice bath is removed and the reactionmixture stirred at room temperature for 3 hours. The reaction is thenslowly quenched with saturated NH₄Cl (10 mL), water (10 mL), and diethylether (20 mL). The resulting product is then extracted with diethylether (3×15 mL), dried over MgSO₄, and concentrated, e.g., using arotary evaporator. The resulting tertiary alcohol (compound 3) is acolorless liquid (4.28 g, 92% yield). It should be noted that Grignardreagents or alkyllithium compounds other than methyl magnesium bromidecan be used for preparation of similar tertiary alcohols.

Preparation of the polymer according to the embodiment in FIG. 3 beginswith an SEBS copolymer (compound VII; 0.50 g, 2.07 mmol styrene units),to which is added compound 3 (1.29 g, 6.22 mmol) in, e.g., a 20 mL vial.The vial is then evacuated and purged with nitrogen.

Anhydrous dichloromethane (5 mL) is added, e.g. by syringe, and thepolymer stirred until dissolved. The vial is then cooled in an ice bathand trifluoromethanesulfonic acid (0.55 mL, 6.22 mmol) is added. Thereaction is stirred in the ice bath for one hour, after which thereaction is poured into methanol to precipitate the polymer.

The polymer is then filtered, redissolved in chloroform, andprecipitated in methanol, yielding the SEBS-alkBr polymer of compoundVIII after isolation and vacuum drying at room temperature for 6 hours.In practice, Applicants found 59.3% of the styrene units of compoundVIII to be reacted (17.7 mol % alkyl-bromide and 12.2 mol %unfunctionalized styrene units). Molecular weights measured by GPC at30° C. with THF as the eluent were SEBS-M_(n)=106,315 g/mol (PDI=1.04)and SEBS-alkBr-M_(n)=60,228 g/mol (PDI=2.07). Viscosities measured intoluene at 30° C. were SEBS=0.82 dL/g and SEBS-alkBR=0.68 dL/g.

Next, 0.15 g of compound VIII is dissolved in toluene (3 mL), filtered,cast onto a Teflon plate, and dried under a gentle flow of air at 80° C.The thin SEBS-alkBr film (approximately 30-40 μm thick) is then removedfrom the plate by immersion in water and immersed in aqueoustrimethylamine (45 wt % in water) and heated to 50° C. for 48 hours. Thefilm is then ion exchanged to hydroxide form by immersion in 1 M NaOH atroom temperature for 48 hours, yielding compound IX of FIG. 3.

FIG. 4 also shows the reactions involved in preparing an SEBS-basedpolymer without use of a transition metal catalyst according to anotherembodiment of the invention. Again, the method begins with an SEBSco-polymer (compound VII; 29.9 mol % styrene units). Compound VII (0.30g, 4.64 mmol styrene units) and 6-bromohexanoyl chloride (compound 4;1.49 g, 6.96 mmol, n=5) are added to a 100 mL round bottom flask undernitrogen. Anhydrous dichloromethane (15 mL) is added, e.g., by syringe.

After stirring to dissolve the polymer, the flask is cooled in an icebath and AlCl₃ powder (0.93 g, 6.96 mmol) is added all at once. Themixture is then stirred in an ice bath for 45 minutes and at roomtemperature for 12 hours. The reaction mixture is then poured intomethanol to precipitate the polymer, which is filtered, redissolved inchloroform, and precipitated in methanol to yield the SEBS-acylBrpolymer of compound X. After vacuum drying at room temperature for 6hours, 0.38 g of the polymer of compound X was obtained, in which 100%of the styrene units were reacted (i.e., the polymer contained 29.9 mol% acyl-bromide).

The ketone of the SEBS-acylBr of compound X may then be reduced to yieldthe SEBS-alkBr of compound XI. To do so, SEBS-acylBr of compound X (0.38g, 0.91 mmol ketone) is added to a 100 mL round bottom flask, which isevacuated and purged with nitrogen. Anhydrous dichloromethane (19 mL) isadded and the solution stirred until the polymer is dissolved.Triethylsilane (Et₃SiH; 0.58 mL, 3.64 mmol) and trifluoroacetic acid(0.56 mL, 7.28 mmol) are added, e.g., by syringe, and the mixturestirred in a 45° C. oil bath. After 14 hours, the reaction is pouredinto methanol to precipitate the polymer, which is filtered, redissolvedin chloroform, and precipitated in methanol to yield the SEBS-alkBr ofcompound XI. After vacuum drying at room temperature for 6 hours, 0.30 gof the polymer of compound XI was obtained in which 100% of the ketonewas reduced.

The SEBS-alkBr of compound XI may then be aminated to yield theSEBS-alkTMA of compound XII. To do so, SEBS-alkBr (compound XI; 0.15 g)is dissolved in toluene (3 mL), filtered, cast onto a Teflon plate, anddried under a gentle flow of air at 80° C. The thin SEBS-alkBr film(approximately 30-40 μm thick) is removed from the plate by immersion inwater, immersed in aqueous trimethylamine (45 wt % in water) and heatedto 50° C. for 48 hours. After 48 hours, the film is rinsed with waterand ion exchanged to hydroxide form by immersion in 1 M NaOH at roomtemperature for 48 hours, yielding the SEBS-alkTMA polymer of compoundXII.

Polymers according to embodiments of the invention may be employed inany number of contexts, including, for example, as fuel cell alkalineexchange membranes, fuel cell ionomers, electrolysis alkaline exchangemembranes, as actuators, and in any number of battery applications, aswill be apparent to one skilled in the art.

One skilled in the art will also recognize, of course, that variouschanges, additions, or modifications of or to the methods describedabove may be made without substantively altering the compounds obtainedor their characteristics. Such changes, additions, and modifications aretherefore intended to be within the scope of the invention.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any related or incorporated methods. Thepatentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal language of the claims.

1. A polymer according to formula IX

wherein x and y are mol % and n is 1-10.
 2. A method of preparing thepolymer of claim 1, the method comprising: immersing a polymer offormula VIII in aqueous trimethylamine; heating the mixture toapproximately 50° C.; rinsing the polymer with water; and immersing theresulting polymer in sodium hydroxide

wherein x and y are mol % and n is 1-10.
 3. The method of claim 2,further comprising: forming the polymer of formula VIII by: mixing apolymer of formula VII with an alcohol of formula 3

adding to the mixture a quantity of anhydrous dichloromethane; coolingthe mixture; adding to the mixture a quantity oftrifluoromethanesulfonic acid; and precipitating the polymer of formulaVIII in methanol, wherein x and y are mol % and n is 1-10.
 4. A polymeraccording to formula XII

wherein x and y are mol % and n is 1-10.
 5. A method of preparing thepolymer of claim 4, the method comprising: immersing a polymer offormula XI in a quantity of trimethylamine; heating the mixture toapproximately 50° C. for approximately 48 hours; rinsing the polymerwith water; and immersing the polymer in sodium hydroxide

wherein x and y are mol % and n is 1-10.
 6. The method of claim 5,further comprising: preparing the polymer of formula XI by: dissolving apolymer of formula X in a quantity of anhydrous dichloromethane; addingto the mixture a quantity of triethylsilane and trifluoroacetic acid;precipitating the polymer of formula XI in methanol

wherein x and y are mol % and n is 1-10.
 7. The method of claim 6,further comprising: preparing the polymer of formula X by: mixing apolymer of formula VII with a quantity of 6-bromohexanoyl chloride;adding to the mixture a quantity of anhydrous dichloromethane; coolingthe mixture; adding to the mixture a quantity of aluminum chloride

wherein x and y are mol %.
 8. A polymer according to formula IV

wherein x and y are mol %, QA is

each of R₁ and R₂ is, independently, a linear alkyl chain or a cyclicalkyl chain, and Z is selected from a group consisting of: a linearalkyl chain, a cyclic alkyl chain, and an alkylene ether chain.
 9. Amethod of preparing the polymer of claim 8, the method comprising:methylizing a polymer of formula III; and carrying out an ion exchangereaction on the resulting polymer

wherein x and y are mol %, QA is

each of R₁ and R₂ is, independently, a linear alkyl chain or a cyclicalkyl chain, and Z is selected from a group consisting of: a linearalkyl chain, a cyclic alkyl chain, and an alkylene ether chain.
 10. Themethod of claim 9, further comprising: preparing the polymer of formulaIII by: carrying out a palladium-catalyzed Suzuki coupling reactionusing a polymer of formula II and an aryl bromide-containing amine

wherein x and y are mol %.
 11. The method of claim 10, furthercomprising: preparing the polymer of formula II by: carrying out aniridium-catalyzed borylation using a polymer of formula I andbis(pinacolato)diboron

wherein x and y are mol %.
 12. A fuel cell alkaline exchange membrane, afuel cell ionomer, an electrolysis alkaline exchange membranes, or anactuator comprising at least one polymer selected from a groupconsisting of: a polymer of formula IV

wherein x and y are mol %, QA is

each of R₁ and R₂ is, independently, a linear alkyl chain or a cyclicalkyl chain, and Z is selected from a group consisting of: a linearalkyl chain, a cyclic alkyl chain, and an alkylene ether chain; apolymer of compound IX

wherein x and y are mol % and n is 1-10; and a polymer of compound XII

wherein x and y are mol % and n is 1-10.